The chancellor staged a tax grab on wealthy homebuyers by raising stamp duty on homes costing £2m or more to 7%, and attempted to slam the door shut on "morally repugnant" tax avoidance by hitting those who buy such properties through a company with a punitive 15% charge.

The 7% rate equates to a £140,000 tax bill on a £2m house, and a rise in the bill on a £5m house from £250,000 to £350,000. Some commentators said the measures meant the Liberal Democrats had more or less succeeded in pushing through a version of their "mansion tax".

To make sure well-off homebuyers actually pay up, George Osborne accompanied his announcement with a warning that he would act "swiftly, without notice and retrospectively" if wealthy individuals or their accountants managed to dream up underhand ways of getting around the rules.

Some property experts were quick to attack the 7% rate as "penal" and "a tax on Londoners" – it is estimated that about 75% of all the £2m properties sold during the past two years are in the capital. But the idea this will cause a slump at the super-rich end of the property market was arguably undermined by figures showing that sales of London houses costing £5m or more have soared by 93% in the first 11 weeks of 2012.

Prices of so-called "prime" central London properties are 11% higher than a year ago, according to estate agent Knight Frank. Some will argue that another £50,000 or £100,000 in tax is small change for a Russian billionaire who sees bricks and mortar in Kensington as a safe haven for his or her money that delivers a decent return.

Since April 2011, stamp duty on £1m-plus homes has been charged at 5%. The 7% rate for properties in excess of £2m will apply from 22 March 2012 – a move the government predicts will raise £150m in the next financial year, rising to £300m by 2016/17.

The Treasury said the 15% rate would apply to properties in excess of £2m bought by "non-natural persons", ie companies. This is designed to combat the practice by some wealthy individuals of "enveloping" high-value properties into companies to avoid paying most of the tax.

Those who have already done this will not escape scot-free. Osborne said he would consult on the introduction of a "large" annual charge on those £2m-plus properties that have already been put into corporate envelopes. And he said that to make sure wealthy non-residents are also caught by these changes the government would be introducing capital gains tax (CGT) on residential property held in overseas companies. He also announced legislation to close down another loophole known as "sub-sale relief" that can allow some people to escape tax.

Accountants PricewaterhouseCoopers said the measures were a clear sign that the chancellor was "throwing the book at avoidance", while rival firm Blick Rothenberg said the announcement that overseas companies that already own UK residential property worth more than £2m will be subject to CGT from April 2013 was a retrospective tax charge. It added: "This can only make the UK less attractive to overseas investors."

Away from the super rich there was disappointment that the two-year stamp duty holiday for first-time buyers, which ends on 24 March, is not being extended. Since March 2010, first-time buyers have enjoyed exemption from the 1% duty on properties between £125,000 and £250,000. For many homebuyers stamp duty is the biggest add-on cost, so this tax break – worth up to £2,500 – is a valuable benefit.

Phil Cliff, director of mortgages at Santander, said: "Getting a foot on the ladder is a tough task for first-time buyers against the backdrop of a sluggish property market. That extra £2,500 can make a huge difference to a financially stretched first-time home buyer, so we are disappointed to see this hasn't been extended by the government."